The present invention relates to an electronic transmitter element which is to be used in a lifting device in order to generate an electrical signal corresponding to the load handled by the lifting device. The transmitter element is provided with strain gauges and is subjected to a tensile strain corresponding to the load. The element according to the invention can either form an integral part of an electronic weighing system or serve as a pure overload protective device, breaking the energy supply to the lifting device when the maximum permitted overload is exceeded.
Transmitter elements of the kind mentioned above are previously known and used in connection with different types of lifting devices such as telfers, cranes, travelling cranes and similar lifting devices. Such arrangements are known for example from SE-A Nos. 7310533-0 and 7502923-1, FR-A No. 2 234 557 and U.S. Pat. No. 3,827,514. There are also transmitting elements intended for weighing purposes or as protection against overload comprising a link-shaped element which is to be used as a splice element in a wire or cable or to support a pulley wheel which is subjected to tensile stresses corresponding to the actual load. These transmitter links must be designed to withstand tensile stresses with good margin relative to the maximum permitted load to be handled by the lifting device. It is common to operate with safety factors in the region of three or four. This means of course that the deformation of the material arising through tensile stresses in the links will be insignificant and utterly uncertain, specifically when the actual load handled by the lifting device is relatively light and in the lower range of the capacity of said device. If the tensile link is to be used as an overload protective device the situation referred to above will of course not have any greater influence but if the link is intended to form an integral part of a weighing system it will naturally mean a considerable drawback. The load detecting elements described in the abovementioned Swedish specifications are in themselves simple strain gauges which are easy to integrate in a lifting device where wires or cables are used for the lifing operation. Such a cable load gauge is also described in U.S. Pat. No. 2,795,136, comprising two cable bearing members in the shape of rollers or pads at opposite ends of the device and a central portion with a tightening device which deforms or bends the cable intermediate the pads or rollers, said tightening device including a link which is subjected to tensile stresses when the cable is subjected to load. These kinds of previously known transmitter elements cannot, however, be used in applications other than pure overload protective equipment and/or for weighing purposes where there is no need for high accuracy.
The object of the element according to the present invention is to accomplish a structure which is both simple and cheap in manufacture and easy to install on site in travelling cranes and other types of lifting devices and simultaneously permits weighing with good accuracy.
This object is realized with a transmitter element substantially characterized in that at least one safety element is arranged in parallel with the tensile link and is designed to withstand higher load than a tensile link, and that said tensile link in normal operation is arranged to absorb the entire load acting on the transmitter element, whereas the safety element in case of rupture or abnormal elongations of the tensile link is arranged to take over at least a portion of the loading acting on the tensile link.
Since there is no need to design the tensile link for loads which substantially exceed the loads to be measured or handled the deformation in the material will be comparatively high and therefore will be detected with high accuracy by means of the strain gauges.
A further object of the invention is to provide a simple installation of an electronic transmitter element in a lifting device comprising at least one pulley wheel which is supported by a frame portion in which the pulley shaft is pivotally mounted in order to generate an electric signal corresponding to the load acting on the pulley shaft. This object is realized substantially by the fact that one end of the pulley shaft is pivotally supported in the frame portion, whereas its opposite end is pivotally supported in the electronic transmitter element, said element comprising a tensile link which is provided with strain gauges, one end of said link being pivotally connected to the frame portion, whereas the opposite end is supporting the pulley shaft.